Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
In recent years, the trend of power converters, including inverters and converters, have shifted to the so-called resonant power converters. Resonant power converters generally employ ZVS (Zero-Voltage Switching) or ZCS (Zero-Current Switching) in order to improve the conversion efficiency by reducing power loss, radiation noise, or transmission noise.
In some cases, the resonant power converter may employ PFM (Pulse-Frequency Modulation) for negative feedback control in order to utilize a portion of the resonant impedance curve representing the property of the resonant power converter or the converter circuit thereof. However, such PFM resonant power converters may not carry out a sufficient control with respect to power supply fluctuations or changes in load that occur in a relatively wide range.
On the other hand, resonant power converters that use PWM (Pulse-Width Modulation) for negative feedback control may provide an easier control than PFM negative feedback control. However, the output of such PWM resonant power converters may sharply decrease or the conversion efficiency of the resonant power converter may deteriorate if a slight mismatch occurs between the resonant frequency and the switching frequency of the resonant power converter.
Various techniques have been proposed to accurately match the switching frequency and the resonant frequency of the resonant power converter by utilizing a PLL (Phase-Locked Loop) in combination with the PWM, PFM or PDM (Pulse Density Modulation).
However, the PLL may operate on a precondition that regular and continuous input pulses are input to a PFD (Pulse-Frequency Detector). In such a PLL, an erroneous operation may be carried out in response to induced noised caused by electrostatic coupling, or electromagnetic coupling, or irregular discontinuities in the input pulses caused by power supply fluctuations or changes in load. The induced noise may include mixing of surge pulses caused by ringing or overshoot. The output of the resonant power converter may sharply decrease if a relatively long recovery time is required to stop the erroneous operation.
The erroneous operation of the resonant power converter may likely occur if the resonant power converter is configured to produce a relatively large output and a PLL part and a power switching part are located relatively close to each other, or a plurality of power switching parts are located on a single substrate or board in order to reduce the size of the resonant power converter.
The decrease in the output of the resonant power converter, caused by a relatively short discontinuity on the order of approximately several hundred μsec to approximately several msec in the input pulses to the PFD of the resonant power converter, may be recovered by a sample-and-hold function of a LPF (Low-Pass Filter) coupled to an output end of the PFD. However, the recovery utilizing the LPF may not be possible if the discontinuity in the input pulses of the PFD is longer, that is, on the order of approximately several tens of msec or longer. A relatively expensive digital signal processing circuit may be required for the recovery in the latter case.